1. Field of the Invention
The present invention relates to an imaging system. More particularly, the present invention relates to a method for developing an image on plain paper wherein a color precursor is associated with an imaging sheet in an image-wise pattern and a film of a developer material is subsequently associated with the imaging sheet to produce an image.
2. Description of the Prior Art
Photosensitive imaging systems employing microencapsulated radiation-sensitive compositions (also known as cylithographic imaging systems) are the subject of commonly assigned U.S. Pat. Nos. 4,399,209, 4,772,541 and 4,842,976 to The Mead Corporation. These imaging systems are characterized in that an imaging sheet including a layer of microcapsules containing a photosensitive composition in the internal phase is image-wise exposed to actinic radiation. In the most typical embodiments, the photosensitive composition is a photopolymerizable composition including a polyethylenically unsaturated compound and a photoinitiator and is encapsulated with a color precursor. Exposure image-wise hardens the internal phase of the microcapsules. Following exposure, the imaging sheet is subjected to a uniform rupturing force by passing the sheet in contact with a developer sheet through the nip between a pair of pressure rollers whereupon the color precursor is transferred to the developer sheet where it reacts to form an image.
Techniques have been disclosed for forming images on plain paper. In U.S. Pat. No. 4,622,282 issued Nov. 11, 1986 a process is disclosed which comprises image-wise exposing an imaging sheet to actinic radiation. The imaging sheet includes a support having a layer of microcapsules containing a photosensitive composition and a chromogenic material on the surface. The sheet is subjected to a uniform rupturing force in contact with a sheet of plain paper. The chromogenic material is image-wise transferred to the sheet of plain paper and the surface of the sheet of plain paper now carrying the chromogenic material is then contacted with a developer material to form a visible image.
In accordance with one embodiment of the invention described in U.S. Pat. No. 4,622,282, the developer material is provided on the surface of the imaging sheet with the microcapsules containing the chromogenic material. The developer may be provided in admixture with the microcapsules in a single layer on the surface of the imaging sheet, but embodiments are also possible in which the developer is provided in a separate layer underlying the layer of microcapsules. With the developer present on the surface of the imaging sheet, images are formed by simply image-wise exposing the sheet to actinic radiation, subjecting the sheet to a uniform rupturing force in contact with plain paper, and effecting transfer to the surface of the plain paper. The developer and the chromogenic material begin to react as they are transferred to the surface of the plain paper.
As disclosed in U.S. Pat. No. 4,622,282, when the developer is not present on the imaging sheet, after transfer of the chromogenic material to the plain paper, the paper is contacted with a developer material and the image is developed through reaction of the chromogenic material and the developer. In one embodiment the paper is contacted with a rotating developer applicator brush. The brush applies a developer material onto the surface of paper where it reacts with the chromogenic material and produces the color image. However, brush application of the developer is disadvantageous as it is often accompanied by streaking, loss of resolution and cross-colorization due to brush contamination as the brush may carry precursor and/or developer across the surface of the paper to undesired areas.
U.S. Pat. No. 4,701,397 teaches another method for forming images on plain paper wherein a layer of microcapsules and developer is area-wise transferred to the surface of a plain sheet of paper to form an image. In this method, the microcapsules and developer are coated in a single layer or as separate layers on a thin polymeric film. The sheet is image-wise exposed to actinic radiation, assembled with a sheet of plain paper and subjected to pressure whereupon, in the image areas, the layer of developer is transferred (selectively adhered) to the paper and an image is formed.
U.K. Patent Application 2 202 641 discloses a method for copying on paper or cloth using the cylithographic system wherein a wax is incorporated in the developer layer. The method relies upon two transfer steps. In the first, the image-wise exposed imaging sheet is assembled with developer sheet wherein the developer is provided on the surface of a temporary support and pressure is applied in the usual manner and an image is formed on the developer sheet. The developer sheet is then juxtaposed a sheet of paper and pressure and heat are applied whereupon the developer layer is transferred as a continuous film to the paper surface.